La Niña, conditions recur every few years and typically last about one year. The one that began in 2007, however, lasted about two years, said study leader Judith Perlwitz of the National Oceanic and Atmospheric Administration (NOAA).

The two-year La Niña affected the patterns of jet streams and and so-called storm tracks across North America.

"If you have colder sea-surface temperatures in the tropical Pacific, they generate circulation patterns in the atmosphere that cause cold air to move into North America," Perlwitz said.

David Easterling, a NOAA climatologist who was not involved in the new work, added that while 2008 temperatures in North America were cooler than average, globally 2008 was still among the warmest on record.

"People often only consider the weather in their locality and not the global picture," Easterling said.

Temperature Match

The team used real-world 2008 sea-surface temperatures to model the atmospheric response and resulting annual surface air temperature in North America in 2008.

The simulated temperatures were a good fit against the actual observed surface temperatures.

For example, both the actual and the computer-model temperature maps showed that northwestern North America was cooler than in previous years, according to the study, published December 8 in Geophysical Research Letters.

The team also examined other possible explanations for the drop, such as volcano eruptions and solar activity.

However, there were no eruptions that could explained the observed cooling.

Peter Stott, the head of climate monitoring and attribution at the Hadley Centre's Met Office, the U.K.'s national weather service, was not involved in the study.

The new research is a convincing explanation of the cold period in North America in 2008, Stott said.

"The study shows quite clearly how the observed pattern of sea-surface temperatures in 2008 led to cooler temperatures than recent years in the United States."

Global warming does not mean every year will be warm, he added.

For instance, scientists often compare temperature patterns under global warming to a craps game in which the dice are loaded: Not every roll of the dice will result in double sixes, but sixes will occur more often than if the dice had not been tampered with.

Humans have, in effect, loaded the climate dice by pumping greenhouse gases into the atmosphere, these experts say.

Natural effects such as La Niña will always ensure that there will be cooler than normal years during a global warming trend, but the definition of "normal" will gradually get warmer as Earth's average temperatures rise, Stott said.

"People adjust to new levels, and there will always be variability," said Kevin Trenberth, an atmospheric scientist at the National Center for Atmospheric Research who also did not participate in the research.

"Global warming does not mean relentless warming year over year and everywhere."